1. Field of the Invention
The present invention relates to an ink-jet recording apparatus for performing recording by discharging ink from recording means onto a recording material. More particularly, the invention relates to a technique for recording an image, such as a photograph image or the like, with high picture quality, and shortening the recording processing time.
2. Description of the Related Art
FIG. 7 is a schematic diagram illustrating a typical conventional technique in which a recording head is mounted and is caused to perform scanning with respect to recording paper, serving as a recording medium.
A carriage 1 for mounting a recording head is fixed to a timing belt 2 for transmitting a driving force to the carriage 1, by a holder 3. The timing belt 2 is tightly stretched between pulleys 4a and 4b, provided at both ends of the timing belt 2. A motor 5, serving as a driving source, is connected to one of the pulleys 4a and 4b. In FIG. 7, the pulley 4a and the motor 5 are coaxially connected.
The two pulleys 4a and 4b are not necessarily identical. Usually, the radii of the pulleys 4a and 4b are determined by the torque characteristics and the velocity characteristics of the motor 5. The idle pulley 4b is used together with a tensioner (not shown) for preventing sag of the timing belt 2, and comprises a member having relatively small inertia. An essential characteristic of this conventional system is one-side driving.
When driving the carriage 1 in a main scanning direction with such a configuration, it is clear that unsymmetrical thrust transmission is performed in a reciprocating operation. This can be simply explained as a difference whether the driving pulley 4a tracts the carriage 1 directly without using the idle pulley 4b, or indirectly via the idle pulley 4b. When performing two-way recording (reciprocating recording), a difference in the elongation property of the timing belt 2 will cause a problem. In addition to a difference in traction of the carriage 1 due to a reciprocating operation, a difference in easiness to induce vibration due to the tension of the timing belt 2 caused by the above-described difference also appears, thereby influencing the result of recording as variations in the speed during carriage scanning. Such variations in the speed will mostly cause stripe-shaped unevenness in the sheet feeding direction in a certain image.
In general, when driving a mass, such as a carriage or the like, according to belt connection, a force directly applied to the mass is the tension of the belt. FIG. 8 illustrates the relationship among tensions applied in the above-described system. The tension between the driving pulley 4a and the carriage 1 is represented by Ta, the tension between the carriage 1 and the idle pulley 4b is represented by Tb, and the tension between the two pulleys is represented by Tc. The position and the direction of each force are indicated by an arrow.
The particularity of a tension is in that the tension has a value of zero or a positive value, i.e., does not have a negative value, and the direction of the force is always constant. In other words, the belt can only pull and cannot transmit a pushing force. The equation of motion of the rigid body of the carriage 1 is determined by the difference between Ta and Tb, and represents acceleration in a dominant direction. If it is assumed that FIG. 8 represents a transient state in which the carriage 1 is accelerated to the right, by revolution of the motor in a counterclockwise direction, the tension increases in the order of Tc and Tb, and, at the same time, Ta decreases. Attention must be paid in that, since transmission of a force from Tc to Tb is slightly delayed, the carriage 1 is first accelerated due to sag of Ta. On the other hand, when the carriage 1 is accelerated to the left, thrust is directly transmitted by the tension Ta.
The sag of the tension is transmitted from Tc to Ta in a delayed state, and interferes with the tendency to increase the tension due to the leftward movement of the carriage. That is, the belt tension Tb between the carriage 1 and the idle pulley 4b has an essential property to induce vibration at a portion where Ta and a colliding action from Tc conflict. FIG. 9 illustrates an example of vibration during acceleration of the carriage 1. Although a large pulsation during acceleration is illustrated in order to facilitate understanding, fine vibration is generated even during constant-speed scanning, thereby causing degradation in the picture quality.
As described above, in the main-scanning driving system of the conventional recording apparatus, since the characteristics of speed control differ in a reciprocating operation, it is difficult to perform high-quality recording in two-way scanning. Furthermore, since the belt tension greatly fluctuates and is directly influenced by pulsation of the torque, variations in the speed of the carriage are large, thereby disturbing the picture quality.